1. Field of the Invention
This invention relates to an angle measuring device, and in particular to an angle measuring device for measuring the turning angle of a crankshaft in an internal combustion engine in order to control the ignition timing, etc. thereof.
2. Description of the Related Art
In order to control the ignition timing, etc. of an internal combustion engine, an angle measuring device is used which is adapted to emit a predetermined pulse in synchronization with rotations of the crankshaft of the engine. FIG. 1 shows an example of conventional devices of this type. The device shown comprises a shaft 1 which rotates in synchronization with the rotation of the crankshaft (not shown), a rotating disc 2 attached to the shaft 1, light-emitting elements 3 and 4, light-receiving elements 5 and 6, and an electronic circuit 7 including a semiconductor signal-processing circuit.
FIG. 2 is a plan view of the rotating disc 2 of FIG. 1. As shown in the drawing, a plurality of slits 8a and 8b are provided at predetermined intervals along the circumference of the rotating disc 2, respectively.
FIG. 3 is a structural drawing of the semiconductor signal-processing circuit in the electronic circuit 7 of FIG. 1. The semiconductor signal-processing circuit shown comprises a wiring layer 9 made of aluminum, an input terminal 10, a P-type resistor layer 11 constituting a protective resistor connected to the input terminal 10, an N.sup.+ layer 12 connected to a circuit power-source terminal 13, a P-type resistor layer 14 constituting another resistor, N-type diffusion layers 15 forming islands, separation layers 16 which separate adjacent diffusion layers 15 from each other, and a P-type substrate 17, one end of which is grounded.
In this conventional angle measuring device, which has the construction described above, the rotating disc 2 shown in FIG. 2 rotates by being driven by the shaft 1, in synchronization with the rotation of the associated crankshaft. Light is input to the light-receiving elements 5 and 6 and causes them to emit signals only when slits 8a and 8b are positioned between them and the light-emitting elements 3, 4. These signals are applied to the input terminal 10 shown in FIG. 3 so that they may be processed. However, as shown in FIG. 3, the resistor layer 11 constituting a protective resistor and the other resistor layer 14 constituting a different element such as a resistor are provided in the same diffusion island 15, so that, when an input voltage which is higher than that of the circuit power source is applied to the input terminal 10, the first P-type resistor layer 11, the N-type layer 12 and the second P-type resistor layer 14 form a parasitic PNP transistor, which causes a base electric current as indicated by the solid arrow and a collector electric current as indicated by the dashed arrow to flow. Thus, electric currents are allowed to flow into other circuits, resulting in the device being subject to malfunction.